Dimensions of the Posterior Tympanotomy and Round Window Visibility Through the Facial Recess: Cadaveric Temporal Bone Study Using a Novel Digital Microscope

Abstract

To measure the width of the posterior tympanotomy in cadaveric temporal bones using the digital microscope and classify the round window visibility through it. In 17 cadaveric wet adult temporal bones, cortical mastoidectomy followed by posterior tympanotomy was performed, delineating the facial and chorda tympani nerves. Antero-posterior width of the facial recess was measured at the levels of oval window and round window with the help of a digital microscope and its software. Visibility of the round window through the facial recess was assessed and classified according to the St. Thomas Hospital classification. The mean antero-posterior width of the facial recess measured was 4.7 ± 0.6 mm at the level of oval window and 4.3 ± 0.7 mm at the level of round window. Round window visibility grading in bones studied were as follows-Type 1 (53%), Type 2a (24%), Type 2b (18%) and Type 3 (5%). Interobserver variability of the posterior tympanotomy measurements using the digital microscope was found to be 91.1% with a 95% confidence interval of 79 to 97% at the level of oval window and 94.1% with a 95% confidence interval (CI) of 87 to 98% at the level of round window. The visibility of the round window is not entirely dependent on the facial recess width at the round window level, suggesting that other factors like cochlear rotation may also contribute to its actual location. Measurements of micro distances with the help of digital microscope seems to be convenient, cost effective and accurate with good inter observer reliability.

Introduction

Posterior tympanotomy, first described by Jansen is the otological surgical approach that provides trans-mastoid access to the tympanic cavity via a triangular space called as the facial recess (FR) [1]. Though superior boundary is the incus buttress and medial boundary is mastoid segment of facial nerve, definitions of classical and extended posterior tympanotomy have varied depending upon whether the lateral boundary of this space is chorda tympani nerve or the tympanic annulus respectively [2]. With regard to cochlear implantation, the width of the facial recess and the exposure of the round window (RW) niche through it are major determinants for insertion of the implant electrodes. Narrow recess has risk of injury to the facial or chorda tympani nerves, whereas a wider recess facilitates easy passage of the implant electrode with the correct angle. Adequate visualization of the round window membrane most often requires drilling of the postero-superior bony niche overhang of the round window and the antero-inferior margin called crista fenestrae. Direct measurements of this small recess are usually difficult, with most of the previous studies in literature employing either measurement on histological cut section slides or on computed tomography (CT) reconstruction images [3–5]. Newer equipment like stereo-microscopes available for the same purpose are quite expensive. The objective of this study was to determine the dimensions of the posterior tympanotomy as well as grade visualization of the round window through it, using a novel digital microscope and assess its reliability and accuracy.

Materials and Methods

This was an anatomical cadaveric study conducted in the temporal bone dissection lab of the otolaryngology department of a medical college hospital. Seventeen wet cadaveric normal adult temporal bones were acquired from the Department of Anatomy and the study was started after approval from the Institutional research and ethics board (IRB No. 11109). The study was conducted over a 1-year duration from July 2018 to July 2019. All temporal bones had a cortical mastoidectomy done followed by a wide posterior tympanotomy that was defined by the posterior limit being the mastoid segment of facial nerve and the anterior boundary being the chorda tympani nerve as in Fig. 1. Measurements and round window visibility were noted with the help of Segolike digital microscope (Cooling Tech Company, China) shown in Fig. 2a. This digital microscope is an electronic microscope having an inbuilt 2 mega-pixel camera with zoom ratio ranging from 50 to 500X zoom, lit by 8 small LED bulbs and powered by connection to a laptop via a USB cable. A specific computer software CD was provided with the device and used to obtain measurements on the images taken by the camera held in hand or fixed on its camera holder (Fig. 2b). Image of a 10 mm distance of a provided scale taken at 100X zoom with the microscope camera was used as the reference to measure the distances on the images after calibration. The various parameters measured included the antero-posterior width of the posterior tympanotomy at the level of oval (OW) and round windows (Fig. 3) as well as round window visibility through the facial recess graded according to the St. Thomas Hospital grading, with maximum table tilt as required [6]. Three otolaryngologists whose experience ranged from 6 to 20 years post-specialization including the primary investigator independently measured the micro distances using the software.

Fig.1.

Round window as visualized through the facial recess

Fig.2.

a Digital microscope, b Measurements made on software

Fig.3.

Arrow lines representing oval window (OW) level and round window (RW) levels of measurements of facial recess width

Statistics

The data were analysed with statistical package for the social sciences, version 16.0 (SPSS Inc,Chicago, USA). Quantitative variables were summarized using mean and standard deviation as in Table 1. Categorical variables were summarized using counts and percentages. Mean length was presented with 95% confidence interval. Intra class correlation coefficient (ICC) was calculated for facial recess width at oval and round windows to check inter-observer variability.

Table.1.

Width of facial recess at oval window and round window along with St Thomas Hospital grade of round window visibility

Bone no.	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	Mean
FR width at OW (mm)	4.8	4.0	3.8	6.0	5.1	4.2	5.2	4.8	4.3	4.8	5.0	3.5	4.9	5.5	5.0	4.1	4.2	4.7 ± 0.6
FR width at RW (mm)	4.5	3.3	3.0	5.7	4.6	4.0	4.9	4.7	4.0	4.3	4.9	3.0	4.5	5.0	4.6	3.9	3.7	4.3 ± 0.7
RW visibility STH grade:	1	1	2a	1	2a	2b	3	2a	2b	1	1	1	1	1	1	2b	2a

FR Facial recess, OW oval window, RW round window, STH St Thomas Hospital

Results

Out of the 17 temporal bones, 11 were of the left side and 6 were of right side. Posterior tympanotomy was carried in all the bones to the maximum possible dimensions keeping the mastoid segment of the facial nerve and chorda tympani nerve intact. The dimensions of the facial recess at the maximum width were measured and this is presented in Fig. 4. Mean width of the posterior tympanotomy at the level of oval window was 4.7 ± 0.6 mm, while the mean width at the level of round window was 4.3 ± 0.7 mm as shown in Table.1. Intra class correlation coefficient for facial recess width at oval window level was found to be 91.1% with a 95% confidence interval of 79 to 97% and at level of round window was 94.1% with a 95% confidence interval of 87 to 98%. The mean width of the facial recess was greater at the oval window level than at the round window level in all the dissected temporal bones. Six bones had width at both oval and round window levels in the range of 4.5-5 mm. In nine temporal bones (53%), round window visibility was 100 and 24% had more than 50% visibility as per St. Thomas Hospital classification. (Fig. 5). In the 3 temporal bones with round window visibility grading of Type 2b, the width of the recess at the round window level was more than the mean width of 4.3 mm, whereas in temporal bone no.7 with the least round window visibility (Type 3), the width at the round window level was 4.9 mm which was again more than the mean. In temporal bone no.3 with the least width at the round window, the STH grading was still type 2a.

Fig.4.

Facial recess width at oval window (OW) and round window (RW) levels

Fig.5.

St. Thomas Hospital grading of round window visibility in this study

Discussion

Cochlear implantation is one of the common indications for posterior tympanotomy along with canal wall up mastoidectomy, facial nerve decompression, excision of middle ear tumours and middle ear implant surgery. An optimal posterior tympanotomy with regard to cochlear implantation is one in which the round window exposure is adequate for easy insertion of electrodes via the round window or a cochleostomy. The width of the facial recess does not vary significantly between children and adults [3, 5, 7, 8]. The width of the facial recess may vary between temporal bones depending on the pneumatisation of the mastoid as well as course of the vertical segment of facial nerve. The ‘slot’ position as described by Ugo Fisch visualizing the tympanic facial nerve segment medial to the incus should be maintained while delineating the facial recess. The tympanotomy drilling commences once a cortical mastoidectomy is completed along an imaginary line drawn from the profile of the short process of incus, parallel to the course of the facial nerve. There are various studies which have attempted to aide safer and effective delineation of this recess, although surgeon’s anatomical knowledge and surgical expertise plays the major role. In a study based on high resolution CT of temporal bone, it was noted that the second genu angle in the oblique sagittal cut was an important factor in predicting the width of the facial recess-greater the angle, wider the recess as the mastoid facial nerve would be more posterior [9]. Kim et al. studied the relation between the vertical facial nerve segment and the mastoid antrum on CT images [10]. Greater take-off angle of the chorda tympani nerve from the mastoid segment of the facial nerve and thereby greater width of the facial recess was observed in pneumatized temporal bones as compared with sclerotic and diploic temporal bones in the study by Jain et al.[11] This study has been a first of a kind attempt to use a digital hand held microscope camera for measurement of micro distances within the temporal bone unlike measurement on CT scans or histology slides. Digital microscopes available commercially are primarily used in the study of insects, coins and microchip engineering. The advantages include a good resolution with zoom option and near to accurate measurements with a computer software. The equipment is also relatively inexpensive compared to the traditionally used stereo-microscopes. The width of the posterior tympanotomy using the digital microscope camera as measured in this study is comparable to the CT based mean measurements previously made by Bettmann et al. (OW level: 5.4 ± 0.9, RW level: 4.5 ± 1.3) and Jeon et al. (RW level: 3.6 ± 0.2) [3, 4]. Our study has also noted an increased facial recess width at the level of the oval window as compared to the round window level as previously noted in other studies [3–5]. This is attributed to the chorda tympani being in a more anterior and medial location at the oval window level as it travels further to enter the middle ear. This wider dimension affirms the fact that it is ideal to start opening the facial recess at a more superior level, just below the short process of incus leaving a ledge of bone (incus buttress) so as to avoid injury to either the facial nerve or the ossicles beneath. We found the inter-observer variability between three surgeons of different post-specialization experience to be excellent—91.1 and 94.1% for facial recess width at oval and round windows respectively as per Cicchetti et al. [12]. This implies that the digital microscope can be used reliably for microsurgical measurements.

Round window visibility is critical for any cochlear implantation as the round window is the portal into the scala tympani of the basal turn. Very often, the widest access into the facial recess does not point directly towards the round window niche [13]. The round window membrane visibility through an adequately made posterior tympanotomy has been classified according to the St.Thomas Hospital (STH) classification after removal of any bony overhang as: Type 1-full exposure, Type 2a-more than 50%, Type 2b-less than 50%, Type 3- not visualized [6]. In one study, it was noted that as the distance between the oval window and round window increases, there is higher chance of difficult visualization of the round window niche, suggesting cochlear rotation as the cause [14]. The anatomical variations between normal cochleae, particularly a difference in the size of the basal turn, as well as developmental changes in the orientation of the cochlear basal turn have also been previously described as contributory factors [15–17]. An overall round window visibility STH grade more than 50% including full exposure (grades 1 and 2a) was present in 76% of the bones which was comparable to a previous study by Leong et al. and Ozturk et al.[6, 18] The same study also recommended that in type 2b, an extended round window approach and in type 3, a separate bony cochleostomy would be needed for electrode insertion. But these options must be attempted after having exhausted all efforts to expose the round window membrane by drilling out any bony overhangs. Also, as noted in this study, lesser width of the facial recess at the round window level does not always necessarily mean difficult visualization of the round window niche, nor does a wide posterior tympanotomy lead to easy exposure of the niche. This further suggests that the round window visibility is not dependant on facial recess width alone and that the operating surgeon has to take into account other factors already described.

Limitations of Study

The sample size of this study is low and hence dissection on more number of bones would have been ideal to bring about more reliable measurements. Secondly, no post-dissection imaging like an CT scan could be done to confirm measured distances. Thirdly, the manual adjustment of the digital microscope camera settings and table tilt required to get the right angulation could have influenced the values measured on the software images.

Conclusion

The mean dimensions of the posterior tympanotomy measured at the level of oval window is 4.7 ± 0.6 mm and at the level of round window is 4.3 ± 0.7 mm. The visibility of the round window is not entirely dependent on the width of the facial recess at the round window level suggesting that other factors like cochlear rotation may also contribute to the actual location of the round window. Measurement of micro distances with the help of a novel device like the digital microscope appears to be accurate, convenient and cost effective and hence useful even in resource crunch settings and may also have a possible application for future studies on the temporal bone.

Funding

Institutional fluid research grant from Christian Medical College Hospital, Vellore district, Tamil nadu, India.

Code availability

NA.

Compliance with Ethical Standards

Conflicts of interest

None.

Ethics approval

Yes (Christian Medical College Institutional research and ethics board No.11109).

Consent to participate

NA.

Consent for publication

Yes.

Availability of data and material

Yes.